Nuclear fusion refers to a type of reaction that occurs when two light nuclei combine to produce heavier nuclei and/or nuclear particles. A small amount of mass is lost in this process. According to the formula of mass-energy equivalence E=mc2, this mass is converted to energy that is eventually converted into thermal energy in the material surrounding the reaction.
These reactions typically occur when a fusion fuel has been heated to a high enough temperature to form a plasma. The temperature at which a plasma under goes fusion varies depending on the type of material. Ignition occurs when a plasma of fusion fuel is heated to a high enough temperature that the fuel becomes hot enough to heat itself through self heating. That is, ignition occurs when the energy released from the fusion reaction exceeds the energy lost through other mechanisms (e.g., Bremsstrahlung radiation). The temperature at which this occurs is called the ignition temperature. For D-T fuel, the ignition temperature can range from 2-10 keV depending on the physical properties of the plasma. After ignition, self heating of the fuel can cause the fuel to rapidly reach ion temperatures of about 100 keV or more. This is often referred to as runaway burn.
Once the fuel has been ignited, confinement refers to the challenge of keeping the fuel from expanding and thus cooling and ceasing to burn long enough to produce the desired amount of energy. The reaction should produce significantly more energy than is used to ignite and confine the reaction. While heating the fuel to ignition is simply a matter of delivering energy to it, confinement is more challenging. Currently there is no way to confine a plasma heated to ignition temperatures or above with a mechanical system. For instance, any solid containment mechanism that comes into contact with the fuel would become instantly vaporized and/or would drastically cool the plasma and quench the burn.